A generally known push-button writing instrument is provided with a refill operating mechanism having a spring biasing a refill slidably inserted in a barrel toward the back of the barrel, and a push-button which is operated to project a writing tip attached to the front end of the refill from an open front end of the barrel and to retract the same behind the open front end of the barrel.
When the push-button writing instrument is held in place in a pocket, it sometimes occurs that the push-button is pushed unintentionally by hand and so on and the writing tip is projected accidentally from the front end part of the barrel. If the writing instrument uses an ink, such as a water-based ink of a low viscosity or an oil-based ink of a viscosity not higher than 3,000 mPa.multidot.s at 25.degree. C., the ink will exude from the writing instrument and the pocket will be stained with the ink.
Recently, there have been provided various writing instruments including push-button writing instruments and using a thixotropic ink generally called a gel ink having a high viscosity when left standing and have a low viscosity when stirred by a rolling ball or the like for writing. The thixotropic ink is contained in a refill without using any ink absorbing wadding or the like which is used for holding a water-based ink. Therefore, if the writing tip part of the push-button writing instrument provided with a refill containing a thixotropic ink is projected accidentally from the front end part of the barrel, and a ball held on the writing tip part is rolled by being rubbed with the fabric of a pocket or the like, the viscosity of the thixotropic ink decreases, and a large amount of the thixotropic ink infiltrates the fabric or is absorbed by the fabric by the capillary action of the fabric.
Accordingly, a push-button writing instrument provided with a refill containing a low-viscosity ink or a thixotropic ink is provided with a safety mechanism capable of preventing the writing tip part of the refill from projecting out of the open front end of the barrel even if the push-button is operated accidentally at least when the push-button writing instrument is inserted and held in a pocket or the like.
However, the push-button writing instrument provided with such a safety mechanism in addition to the refill operating mechanism has a complicated mechanism, and needs additional parts. Therefore, it is difficult to employ such a safety mechanism in a low-price push-button writing instrument in view of problems in manufacture.
In view of such problems, the inventors of the present invention made studies to provide a push-button writing instrument provided with a refill operating mechanism analogous to the conventional refill operating mechanism, and capable of preventing a writing tip part of the refill from being projected out of the open front end of a barrel when a force on a touching level is applied to the push-button. The inventors found through the studies that the pressure necessary for operating the refill operating mechanisms of the conventional push-button writing instruments is in the range of 200 to 300 grams and the accidental projection of the writing tip part of the refill from the open front end of the barrel by an inadvertent touch to the push-button can be avoided by increasing the pressure necessary for operating the refill to project the writing tip part of the refill from the open front end of the barrel, and have made the present invention.